Ethylene, an olefin compound is one of the key petrochemical compounds manufactured in large quantities around the world. It is used to manufacture consumer products like plastics, tires, automobile antifreeze, etc. While ethylene industry began by recovering ethylene from refinery by product gases, thermal cracking has become the main production method today. Over the years technology has evolved so that ethylene is produced by thermally cracking full spectrum of hydrocarbon streams such as ethane, propane, butanes, naphtha, gas oil etc.
Thermal cracking of hydrocarbons to produce ethylene is beneficial at low pressures and low reaction time. Thermal cracking of hydrocarbons is done in radiant tubes as shown in the FIG. 1. Low hydrocarbon pressure is achieved by diluting hydrocarbon stream with steam. Higher the amount of diluting steam, better the production of ethylene. Similarly smaller the tube volume, better the ethylene production. Amount of steam used in industrial practice is determined by the economics. Usually ratio of diluting steam to hydrocarbon feed ranges from about 0.3 for ethane to about 1.0 for gas oil. Desired reaction time is achieved by providing suitable combination of tube diameter and tube length. Reaction times have evolved over the years so that we can design tubes with reaction time of about 0.1 second compared to more than 0.5 seconds in the old days. Lower the reaction time higher the reaction temperature, termed as coil outlet temperature (“COT”).
Another important process parameter in thermal cracking is the conversion of hydrocarbon feed. Higher the conversion of hydrocarbon feed higher the production of ethylene. However, higher the conversion of hydrocarbon feed, higher the coil outlet temperature. Since the amount of dilution steam is limited by economics technology has evolved around increasing hydrocarbon conversion and lowering reaction time. For example, thermal cracking of ethane at 55% conversion and 0.5 seconds reaction time with dilution steam to ethane ratio of 0.3 occurs at a theoretical temperature of 819° C. compared to ethane at conversion of 65% and reaction time of 0.1 seconds with same dilution steam to ethane ratio of 0.3 occurs at a theoretical temperature of 884° C., a rise of 65° C.
An industrial ethylene furnace 10 is shown in FIG. 1. Ethylene furnace 10 is divided into two sections, namely, a radian section 12 and a convection section 14. Radiant section 12 contains a bank of tubes termed as radiant coils (not shown), heated by a firing fuel 18 on the outside to achieve a desired coil outlet temperature 20 at outlet 22 for a given hydrocarbon feed and steam mixture. A hydrocarbon feed 24 flowing through valve (F1) 25 25 is mixed with a dilution steam stream 26 to form a mixture 28 that enters radiant section 12 at an incipient reaction temperature 74 and exits at a predetermined coil outlet temperature 20. For example ethane and dilution steam mixture will enter radiant section 12 at about 675° C. Flue gases from radiant section 12 exit to convection section where the heat contained in the flue gases is primarily used to heat hydrocarbon feed and steam mixture to incipient reaction temperature and balance is used to generate utility steam. Obviously incipient reaction temperature and coil outlet temperature varies with feedstock type and desired conversion.
Thermally cracked hydrocarbons termed as cracked gas and steam mixture exits radiant coils at 22 and are quenched very quickly to arrest cracking reaction at a quenching device. The quenching device is preferably a shell and tube type exchanger shown in the FIG. 1, referred to as a transfer line exchanger (“TLE”) 34, where water is circulated on the outside of tubes. Alternately, a quenching device can be a double pipe exchanger, which is somewhat expensive. The quenched cracked gas and steam mixture exits transfer line exchanger 34 at 36 and is sent to a recovery section via a line 40 where ethylene and other products are separated. Heat recovered in transfer line exchanger 34 enhances utility steam generation when combined with heat available in convection section 14 to generate utility high pressure steam 42. Normally there are quite a few radiant coils and correspondingly appropriate number of transfer line exchangers. The method described and claimed in this invention can be performed in any ethylene furnace 10 having any type of radiant coil coupled with any type of quench exchanger.
Thermal cracking of hydrocarbons produces many by products along with coke. Coke adheres to radiant coil walls and tube walls of quench exchangers. As coke builds up on the radiant coils, efficiency of heat transfer from flue gases to hydrocarbons in the tube goes down resulting in higher and higher tube metal temperature for constant coil outlet temperature. Second consequence of the coke build up is increasing pressure drop in the radiant coils and transfer line exchanger. Ultimately a limit is reached on tube metal temperature and/or pressure drop when production must be stopped. The time between the start of production to shutdown of production due to coking is termed as furnace run length. Observed furnace run lengths are in the range of 5 to 60 days. Coking increases rapidly at higher coil outlet temperatures. Since thermal cracking technology has evolved to higher and higher coil outlet temperatures coking becomes increasingly severe as well as critical problem. Coking in transfer line exchangers can be partly due to coking and partly due to condensation of tars on the relatively colder tube walls.
In order to prepare ethylene furnace 10 for decoking, hydrocarbon feed 24 is cut off and dilution steam flow may be increased by increased flow of dilution steam 26 and/or flow of dilution steam through a line 27 maintaining a predetermined coil outlet temperature 20 with furnace steam going to the recovery section. This phase of the furnace operation is termed as “Hot Steam Stand By” (HS SB). After HS SB, valve (V1) 50 is closed and valve (V2) 52 is opened so that furnace steam will be routed to “decoke system”. Once closure of valve (V1) 50 is completely confirmed, air is introduced into furnace 10 to start combustion of coke. Steam-air decoking procedures have been patented by Lohr, et al. U.S. Pat. No. 4,376,694, (1983) Sliwka et al. U.S. Pat. No. 4,420,343 (1983) and De Haan et al. U.S. Patent Application 20090020459. In principal, combustion of coke is monitored by measuring CO2 in furnace 10 effluents. Combustion of coke is controlled by amount air mixed with steam. Initially air to steam ratio is small and it is increased slowly based on effluent CO2 content. When effluent CO2 content falls below one mole % on dry basis, coil outlet temperature is increased so that hotter steam air mixture can “vaporize” condensed tars in TLE's. Some operators perform Transfer line exchanger tar vaporization phase with air alone. Transfer line exchanger decoking may take about 15 to 30 hours. The decoking process is terminated when effluent CO2 content falls below a predetermined level such as 0.1 mole % on dry basis and/or Transfer line exchanger outlet temperature stabilizes. First step after termination of the decoking is cut off air flow to furnace 10, and only steam is allowed to flow with valve (V1) 50 still closed and valve (V2) 52 open. When stoppage of air flow to furnace 10 is completely confirmed valve (V1) 50 is opened and valve (V2) 52 is closed thereby isolating “decoke system” and running furnace 10 in HSSB mode. The decoking period can vary from 12 hours to 48 hours. When furnace 10 is running in HSSB mode, predetermined rate of sulfur compound is injected into steam to passivate the radiant coils. Sulfiding may be done in 4 to 6 hours. When sulfiding is complete then furnace 10 is ready for feed introduction. This entire process is recognized as “steam-air hot decoke” method and is employed by almost all (99%) producers (van Helmond, 2009). During decoke TLE's operate at low temperatures where coke combustion is very poor and vaporization of tars is difficult to complete. As a result it becomes necessary to clean TLE's mechanically after 3-4 hot decoke cycles.
Steam-air decoking method has consequences. Since coke is combusted inside the radiant coils local hot spots are created leading to severe carburization. Carburization is greatly increased at high temperatures with low or zero amount of steam flow as employed in Transfer line exchanger decoking phase (Tillack, 1998). Very aggressive steam-air decoking is believed to be a major cause of carburization. The most helpless fact of carburization is its non-uniformity and unpredictability. Carburization is the major cause of failure of radiant coils. Difficulty in controlling air flow is described by De Haan et. al. in U.S. Patent application 20090020459.
Steam-air decoking has undesired side effect of cracking radiant tubes when the “hot spots” are uncontrollable. By the time hot spot is observed it is too late to control furnace 10 firing. Failure of radiant coils due to cracking increases maintenance costs.
Steam-air decoking method has further consequences. Due to the use of air in decoking process, metal sites on radiant coils become very active and cause high coke and carbon monoxide production (Zisman, 2000). These active sites on radiant coils need to be passivated using sulfur compounds during HSSB phase. Amount of sulfur dosage required for sulfiding can be very high. Also as radiant coil ages, tube sites become more active and require higher and higher amount of sulfur for passivation. As like carburization, sulfiding is non-uniform and unpredictable. McKimpson and Albright (2004) raise the question that can radiant coil exit, where coking occurs the heaviest, be properly sulfided? They observe that most sulfur is reacted before it reaches the radiant coil outlet portion. Improper sulfiding can lead to very high rates of coke formation during normal cracking operation.
Sulfiding has further consequences. During sulfiding phase steam is routed to recovery section via valve (V1) 50. Sulfur compounds used in the sulfiding phase can produce acidic compounds which lower quench water pH. Furthermore some sulfur compounds behave like surfactants producing a stable emulsion of quench water and gasoline which results in fouling of equipment in quench water and dilution steam circuits (Zisman, 2000, De Haan, 2006). The most unpredictable and undesirable consequence is that the dilution steam contaminated with gasoline coming to furnace 10 will cause fouling in furnace 10 convection section 14.
Sulfiding has more unintended consequences. Ethylene plants may have gasoline hydrogenation unit especially if the main hydrocarbon feed is naphtha or gas oil. Sulfur compounds injected in the sulfiding phase, if unconverted, can end up in gasoline. Sulfur in gasoline will poison gasoline hydrogenation catalyst. (De Haan, 2006).
Alternative to steam-air decoking has been the steam-only decoking procedure (Zimmermann, 2005) which is now almost a forgotten art. In steam-only decoking water gas reaction takes place to gasify coke. Water gas reaction is endothermic which does not cause hot spots in the radiant coil. As a result carburization is reduced greatly compared to steam-air decoking. Furthermore passivation of radiant coils is not required there by eliminating sulfiding altogether and its bad side effects as described above. Those who have been able to manage steam only decoking have experienced longer radiant coil life. However steam only decoking fell out of favor because it was slow even when employing difficult to manage high coil outlet temperatures of 1000° C.
Ethylene industry would greatly benefit if steam-only decoking can be made faster without significant bad side effects so that it can compete effectively with steam-air decoking and ultimately replace it.
Accelerating water gas reaction to gasify coal or coke is probably the most investigated subject matter. Application of chemicals to gasify coke in ethylene radiant tubes when furnace 10 is on line, i.e. processing hydrocarbons, has been tried. For example, Kohfeldt and Herbert in U.S. Pat. No. 2,893,941 in year 1959 proposed injecting aqueous solution of potassium carbonate (K2CO3) into gas oil and steam mixture being cracked. Coke produced by gas oil cracking in radiant coils, will gasify by potassium carbonate (K2CO3). Kohfeldt and Herbert reported success in extending furnace run length. Kohfeldt and Herbert also observed that potassium carbonate (K2CO3) reduced coking in convection section 14 banks which were operating at temperatures less than 400° C. Kohfeldt and Herbert's methodology is recognized as on-line coke control since effluents are routed to recovery area. However, this methodology was never adapted by ethylene industry probably because it would have adverse effects in the recovery area operations downstream e.g. affecting pH of quench water etc. Recently Gandman in U.S. Pat. No. 6,228,253 proposed injecting aqueous mixture of potassium carbonate and magnesium acetate in ethylene furnace radiant coil or coils where steam decoking effluent along with cracked effluents from other coils were routed to recovery area similar to Kohfeldt and Herbert. Gandman reported success in removing coke from the selected radiant coils. Stancato and De Haan in 2001 reported at the Ethylene Producers Conference, that rate of gasification of coke by Potassium carbonate (K2CO3) is 16 times the rate of gasification by steam alone.
Main products of water gas reaction are CO and H2 and along with a small quantity of CO2. If water-gas reaction products are sent to recovery section during decoking CO concentration will spike in the cracked gas. CO is a poison to downstream hydrogenation catalysts. Apparently some companies have tried to market variation of water-gas accelerants for on-line coke control without wide acceptance. Obviously, risk of poisoning downstream hydrogenation catalyst is too great for producers.
Proper application of water-gas reaction accelerants is in the steam decoking step where decoke effluents are routed to “decoke system” there by completely avoiding risk to recovery area operations. However, this can be accomplished only if one is versed in process parameters used in steam-only decoking. Proper marriage of steam decoking process parameters such as steam flow rate and coil outlet temperature with chemical injection rate is required to achieve success.
Patents and literature are full of many investigations of chemicals used in accelerating water-gas reaction. We researched literature to find another reasonably priced chemical which can provide synergy with potassium carbonate (K2CO3). In a comprehensive review of carbon gasification, Nand (1981) in his thesis refers work of Kayembe and Pulsifier (1976) who found that the highest steam gasification rates were achieved by potassium carbonate (K2CO3) and Potassium hydroxide (KOH).
According to the present invention, an efficient and economic process for decoking ethylene furnaces or for reducing the rate of coke deposition in ethylene furnaces is disclosed. These and other advantages of the present invention will become apparent from the following description and drawings.